Sunday, 30 November 2008

Some time ago I blogged that the GM3 gearmotor generates a lot of RFI, which was interfering with TV reception in our house and corrupting I2C comms on HydraRaptor. I designed a simple suppressor that fixed the problem, details here.

Recently Zach Smith designed a nice little PCB version of it and produced a kit. He gave me a sample to test. Here it is installed on a GM3: -

To test it I wired a GM3 with no suppressor to a bench power supply with a pair of jumper cables about 30cm long. I viewed the noise on both motor terminals with a scope grounded at the PSU. This is what it looks like in the time domain.

It is massively noisy producing about 50V pk-pk. And here is the spectrum in the frequency domain: -

Although this is the 12V version of the motor it looks similar to the 6V version I tested before.

I repeated the same test with the suppressor fitted, measuring the voltage at the terminals of the suppressor.

The noise is vastly reduced, now only about 700mV pk-pk.

The spectrum is reduced drastically as well: -

Compared to my Vero board version, tested under the same conditions, it seems to work a bit better, but that could be down to variations between motors.

So the kit version works well and also gives convenient screw terminals or 0.1" header robustly anchored to the motor.

Tuesday, 25 November 2008

I fancied making a dodecahedron, an object with twelve pentagonal faces. It is an interesting shape and, as the sides slope at ~26°C, it can be made without support material. I searched the web for a 3D model for some time but could not find one. I also searched for how to model one in CoCreate, as it wasn't immediately obvious to me. That came up blank as well so I had to figure it out myself.

I started with a construction circle and divided it into 5 sectors with construction lines 72° apart. I joined the intersections to make the base pentagon.

I then extruded that to a height equal to the circle radius and with a draft angle of -26.56505°. This is the dihedral angle (2arctan((1+√5)/2)) minus 90°. That makes the base of the object and the first line of vertices above it.

I then made a new workplane on one of the partial faces. I projected the face onto the workplane and then added a construction circle through three of the points. A vertical line from the centre gives the missing fifth vertex where it meets the circle.

I then join the vertices to make the pentagon, extrude it inwards (negative) by the circle radius with the same negative draft angle.

That operation has generated two partial faces with all five vertices. I construct the pentagons from the vertices and extrude inwards by the circle radius until the shape is complete. A total of eight extrusions are required.

I then shelled the object to 2mm to make it hollow. That created a second part inside, revealing that the construction does not in fact make a complete solid. If that was important one could extrude one of the faces more than half way through, with no draft angle. I just deleted the second part.

The finished item is about 2.5 times initial circle radius across opposite flats. This one was based on a 10mm radius circle.

Today I noticed that each tools is stamped with a DIN standard. Googling those mainly came up with German and Scandinavian adverts for the tools, some of them charging $99 for one tool! Eventually I found a German site selling DIN standards. Putting in each number gives the title of the standard, which is the description of the tool.

So from left to right we have: -

DIN 4974 R Internal side turning tools for corner work with carbide tips.

DIN 4973 R Boring tools with carbide tips.

DIN 4980 L Offset side turning tools with carbide tips.

DIN 4980 R As above but right handed.

DIN 4978 R Offset turning tools for corner work with carbide tips.

DIN 4971 R Straight turning tools with carbide tips.

DIN 4972 R Bent turning tools with carbide tips.

DIN 4977 R Offset face turning tools with carbide tips.

DIN 4981 R Parting-off tools with carbide tips.

DIN 4976 Wide face square nose tools with carbide tips.

DIN 4975 Pointed straight turning tools with carbide tips.

1 & 2 are mounted parallel to the lathe axis and are used to bore out the inside of the workpiece. Each cut would be along the axis and successive cuts would move outwards.

8 would also be mounted parallel to the axis, but with the cut going across the face of the workpiece.

9 is mounted perpendicular to the axis and is driven into the work to cut it off from the stock. I hadn't realised it was a parting off tool, I have happily been using it to turn bearing lands with straight sides. I bought a thinner parting off blade, which wastes less material.

All the other tools are for mounting roughly at right angles to the axis and cutting along the outside of the workpiece.

11 has a 60° point for cutting external threads. I bought a set of gearwheels for cutting metric threads. These alter the speed of the lathe's auto feed to match the thread pitch. The procedure looks a bit complicated though, so I have stuck to using a die so far. It would be nice to be able to make large pitch ACME lead screws but that seems even more complicated.

I have had titanium nitride indexable tools recommended to me for their very long life. They have a three pointed replaceable tip. Each time a tip wears out you can rotate it until they are used up, at which point you replace it. They are quite expensive but replacement tips are not too bad.

You can also buy blank steel tools and grind them to whatever shape you want yourself.

Tools are mounted in the tool post and held down with machine screws :-

The tip of the tool must be at the same height as the centre of work. This is most easily done by aligning it against the tailstock centre. Small differences can be compensated for with the rocker under the tool by tilting it slightly. Large differences would result in the tool at the wrong angle so the other side of the post is used with a shim. The tool should be gripped as close to the working end as possible to prevent chatter.

Compound SlideThe lathe comes with a cross slide that allows you to move the tool across the workpiece and the feed screw allows motion along the axis of the lathe. To be able to cut a taper, and for cutting threads, you need to be able to move at an angle to the lathe's axis. The compound slide replaces the tool post and adds another axis of movement at any desired angle.

The next model up lathe, the CL300M, includes a compound slide, so if you take that into account the price difference is not that much. A lathe without one is quite limited, IMHO.

Drill ViceI got a tiny quick release drill vice that fits on the cross slide to hold work for drilling or milling. Quick refers to the fact that rather than screw the vice all the way to open it, you lift a ratchet and drop it in the nearest slot for the width you want. Then you use the screw to tighten it no more than a quarter of its maximum travel.

This highlights the main compromise having the lathe, mill, drill combo. The cross slide is wider than normal for a lathe, which limits how close the tailstock can get to the chuck, but small for a milling table or a drill table. Also the height of the cross slide means that the tallest thing you can drill is not as much as you would expect from a pillar drill. If I get desperate I could remove the tailstock and cross slide, move the saddle out of the way and put a board on top of the lathe bed to drill a large object.

Tailstock ChuckThis is just a drill chuck with an MT1 Morse taper to fit into the tailstock quill. It allows you to drill into the center of the work piece. You might think holding the drill stationary and spinning the work is the same as spinning the drill and holding the work. It isn't, the former ensures the hole is exactly down the centre of the workpiece.

The chuck is exactly the same as the one the comes with the milling machine, but annoyingly that and the headstock have MT2 tapers but the tailstock is only MT1. If the tailstock was MT2 it could have shared the chucks with the milling machine, and also many other tailstock attachments, like boring heads and tap holders that only seem to go down to MT2.

Four Jaw ChuckThe lathe comes with a three jaw self-centring chuck. The jaws all move together to hold a round or hexagonal workpiece centrally (within the accuracy of the chuck). To hold a square or octagonal shaped object you need a four jaw chuck. These usually have jaws that move independently allowing / requiring objects to be centred manually. That also allows rectangular objects to be centred and you can also mount things deliberately off centre.

Potentially you can centre round things more accurately than you can with a three jaw chuck but it takes more time and skill.

Large, and or completely irregular shaped things can be held using a faceplate and bolts or clamps.

Milling ChuckThe drill comes with a normal three jaw drill chuck.

A collet chuck provides better centring and grip for milling. Expensive, but the sizes match a set of end mills I already had.

This also fits the headstock, so another way you can mill and drill on a lathe is by using a vertical slide.

WigglerThis is a special chuck and a set of probes that are used for lining up the drill / mill with centre marks and edges of the workpiece.

The special chuck forms a ball and socket joint with the probe so it can swing. When you spin it in the drill chuck it rotates in a circle, but by pressing on the edge you can persuade it into a mode where it spins dead centre to axis of rotation. You use the point to line up on centre punched holes and the ball and cylinder for finding edges. The bent one is used with a dial indicator but I don't understand how or why.

Instructions for using one are here. The balls are imperial sizes, which is a pain if you are working in metric units. I don't know if you can get metric ones.

Die HolderThis allows tapping an external thread by turning the workpiece in the chuck and holding the die with the tailstock.

The bar goes in the tailstock. The tube slides over it and can be rotated part of a turn with the handle. Different size dies can be used in each end of the holder and the two adaptors allow for two larger sizes.

I could not find one to fit an MT1 tailstock, so I had to get an MT2 one plus an MT1 to MT2 adaptor.

This is far from ideal because it takes up so much of the distance between centres. I ground off the tang at the end of the taper because there is nothing to engage it on my lathe. That gives me about another 10mm. I will probably bore out the end of the die holder's bar so that it will accept an M5 bar inside it. That will allow longer threads to be tapped without weakening it too much.

One other problem with the die holder was that the set screws in it do not have pointed ends. When using split dies the middle screw should have a cone shaped point so that tightening it forces the split open, allowing an oversize thread to be cut. Tightening the outer two makes an undersized thread. When tapping something hard like stainless steel you need to start oversized and then work down.

I solved the problem by turning a point on an M5 setscrew to replace the middle screw.

I should really have made one of these, I think, rather than buying this one that is too big. It should be fairly straightforward to make on a lathe. I don't fancy turning an accurate MT1 taper but you can buy Morse tapers with a soft blank on the end for machining to a custom use.

Centres

The MT1 tailstock centre of the right came with the lathe and is known as a "dead centre" because it does not rotate. The one in the centre is a "live centre" because it has a bearing, which allows it to rotate with the work, reducing the friction. There is also a variant called a "half dead centre" which is a dead centre with half the cone cut away to allow a facing tool to get in.

Friday, 21 November 2008

I came across this object designed by Gorg Huff in the RepRap objects wiki. It was such an interesting organic shape, completely different from anything else I have printed, that I had to try it.

It was a bit too big for my machine so I scaled it down and printed it diagonally.

It took about 4 hours plus an hour for the raft. Because the sides slope in quite quickly, Skeinforge switches to 100% fill for a lot of the layers because the edges don't have anything two layers above them. This can be fixed by selecting 3 extra shells on sparse layers. That means the infill starts far enough from the edge to have something two layers above it. You get a stronger object with less plastic that way.

Thursday, 20 November 2008

When I was making the chuck grip I noticed that the raft changed colour part way across the top layer of the raft.

The heater seemed to be on 100%, so it looked like the plastic was way too hot. By the time I noticed it seemed to have reached thermal equilibrium and apart from some snap crackle and pop sounds, and a bit of smoke the extruder seemed happy. I was reluctant to abort the build because it had taken about an hour to get this far.

When it finished the raft it cooled down to the right temperature and built the object. The surface of the raft has a completely different texture and it seemed easier than usual to peel off the object. Despite that, it managed to hold down what was a very big object. The shape of the object was less prone to curling than most, being a large circle (no corners to curl up) split into three segments and with a corrugated outside perimeter, which could absorb shrinkage. I need to do more experiments to know if it is beneficial to deliberately make a raft like this.

This is what the normal and hot rafts look like close up :-

And here they are under a microscope :-

To investigate further I ran a test with the heater target temperature set to 300°C and monitored the thermistor reading. It maxed out at 290°C. That is fortunate as it is just below the point where PTFE is supposed to start decomposing into poisonous substances. For some reason the PTFE holds up mechanically, I would have expected the barrel to pop out. Perhaps the ABS becomes so fluid that there is very little pressure required for extrusion. Anyway, the extruder seems happy operating at 280°C, where it just about manages to control the temperature with 96% PWM.

The filament changes from green and smooth to almost cyan and a rough texture: -

Again under the microscope the surface looks very different :-

My theory as to what is happening is that the green dye is composed of yellow and cyan dyes, and the yellow component is boiling off, disrupting the surface.

I had a go at making some objects at 240°C, 260°C and 280°C :-

It seems that 240°C is about the limit for green ABS before it starts to change colour and texture. The bottom of each object has to be at the correct temperature so it can be separated from the raft but other layers could be chosen to be different temperatures to give a stripy effect. The hot objects seem very strong and feel like they wont de-laminate in a hurry.

I don't think you can keep the plastic long at those temperatures, I found this mess under a raft. I think the temperature had gone wrong during warm up.

Initially I had no idea why my temperature control was occasionally going wrong. The thermistor is still well attached. I caught the effect with some logging and discovered that the temperature was reading about 40°C low some of the time. Touching a connector seemed to fix it. I could not find a loose connection so I just unplugged it and plugged it in again. I has been OK since. With a 10K thermistor you only need a few ohms to make a big difference at the high end.

So an interesting effect that might be exploitable for support material or aesthetic effects.

Monday, 17 November 2008

Zach Smith and Bre Pettis have created a web site called Thingiverse, which is designed to make it easy for people to share digital designs of real objects. I have put most of the things I have designed for printing on a RepRap on there. I even created one especially for it, a key for reading utility meters :-

You can get these free from the utility companies, but not a quickly as you can RepRap one for a few pennies worth of plastic, and they are easily lost. The files can be found here :- www.thingiverse.com/thing:88.

A lot of the objects on Thingiverse are for a laser cutter, but I downloaded this twisted star box designed by Marius Kintel and printed it.

One trick I have learnt is that you can shrink objects a little by heating them with a hot air gun. When I first made this the top was too tight, so I shrank the bottom a little with the heat gun. It is now a perfect fit. You have to be careful not to get it too hot, or it will sag.

Tuesday, 11 November 2008

The company that I have worked at for 25 years gave me a long service award recently. I could choose anything worth £500 so I chose this small lathe / drill / milling machine combo. The tiny watchmaker's lathe I have been using up to now is not really big enough for RepRap parts. This should be just about right.

A combo like this is a bit of a compromise and only recommended if, like me, you have limited space. It gives me a lathe, pillar drill and milling machine in a small footprint.

I also bought accessories with my own money, which came to about another £300 :-

Cutting tools - essential.

Compound cross slide - for cutting tapers.

Drill vice - to hold things on the cross slide when milling or drilling.

Tailstock chuck - to drill down the axis of round things.

4 Jaw chuck - for turning square and irregular shapes.

Milling chuck and collet set - to hold milling bits.

Wiggler - for finding centres and edges when milling and drilling.

Die holder - for tapping threads.

Headstock centre - for turning between centres.

I have also made a couple of accessories with HydraRaptor. The first is very simple: a t-slot cleaner for removing swarf from the cross slide's t-slots.

That saved me about £2.80.

The second object is the biggest thing I have RepRapped so far. It is a cover to go over the chuck to make it easier to turn by hand when tapping. It took over 7 hours to build and weighs 77g.